As an example, I am looking for a mod_files.sh file which presumably would come with the php-devel package. I guessed that yum would install the mod_files.sh file with the php-devel x86_64 5.1.6-23.2.el5_3 package , but the file appears to not to be installed on my filesystem. How do I find out which package installs the specific file I'm looking for where I have not necessarily already locally downloaded the package which may include the file that I'm looking for?

Well finding the package when you are connected to internet (repository) is easy however when you only have access to RPM packages inside Redhat or Centos DVD (this happens frequently to me when I have to recover a server and I need an application) I recommend using the commands below which is completely independent of internet and repositories. (supposably you have lots of uninstalled packages in a DVD).
Let's say you have mounted Package folder in ~/cent_os_dvd and you are looking for a package that provides "semanage" then you can run: